% http://tex.stackexchange.com/questions/2326/vertically-center-text-on-a-page
\begin{frame}[plain, c]
    \topskip0pt
    \vspace*{\fill}
        \begin{center}
            \Huge \textbf{Fragen?}
        \end{center}
    \vspace*{\fill}
\end{frame}